The elastic modulus matrix of an HTI medium with a symmetry axis along the x direction (which I will denote as ) has the following form (in compressed notation):
From here on, to simplify notation, I use the variables and , introduced by Dellinger 1991. Thus we rewrite equation (4) as
Figure 1 Definition of propagation angles. The azimuthal angle is measured with respect to the symmetry axis, and the incidence angle with respect to the vertical axis.